300E 2.6 and 300E: Differences in engine?
 

Are both M103 engines, is one more reliable than the other, do they have the same issues, and is 2.6 a lot less powerful?

The 2.6 I believe had MB Tex instead of leather, and a couple of other subtle differences. The engines are both M103 with different displacements and are both bulletproof. With that said, they also share the same common issues such as OVP, front U-seal, etc. The 2.6 is much less powerful and I don't think it is much more fuel economical.